Annular electrodes in differential pumping tubes for electrostatic accelerators

ABSTRACT

1,087,725. Particle accelerators. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Nov. 11, 1965 [Nov. 19, 1964], No. 47158/64. Heading H1D. A differential pumping tube arranged in parallel with a D.C. particle accelerator tube and having potential distributing annular electrodes 1 along its length has each electrode comprising a diametral bar 4a, 4b bent out of the plane of the electrode, all the bars being bent towards one end of the tube. An electric field having a component towards the axis of the tube is thereby set up between pairs of adjacent electrodes in order to suppress electron loading. The bars may be bent to V- or arcuate shape and successive bars may be circumferentially displaced through a small angle. Also the inner periphery of each electrode may be bent from the plane of the electrode.

Sephl9, 1967 F. A. HOWE ETAL 3,342,404

ANNULAR ELECTRODES IN DIFFERENTIAL PUMPING TUBES FOR ELECTROSTATICACCELERATORS Filed Nov. 12, 1965 United States Patent 3,342,404 ANNULARELECTRODES IN DIFFERENTIAL PUMPING TUBES FOR ELECTROSTATIC ACCELERATORSFrederick Albert Howe, Newbury, and Ronald Inch Bell, Basingstoke,England, assignors to United Kingdom Atomic Energy Authority, London,England Filed Nov. 12, 1965, Ser. No. 507,426 Claims priority,application Great Britain, Nov. 19, 1964, 47,158/ 64 5 Claims. (Cl.230-69) ABSTRACT OF THE DISCLOSURE A differential pumping tube forevacuating the highvoltage portion of an electrostatic accelerator inparallel with the accelerating tube is of the type comprising aplurality of annular electrodes spaced apart by annular insulators, andis improved by including in each electrode a substantially diametricalbar which intersects the axis of the tube, the two radial halves of thebar being deflected from a plane normal to the tube axis toward one endof the tube to produce an electric field between adjacent electrodeshaving a component directed toward the tube axis to cause deflection ofunwanted electrons into the electrodes. The inner periphery of eachelectrode may also be deflected from the plane toward the one end of thetube to enhance the electric field, and preferably each successiveelectrode has its bar circumferentially displaced through a small anglerelative to the bar of the preceding electrode to provide opticalbaffling along the tube.

This invention relates to dilferential pumping tubes for electrostaticaccelerators.

In most Van de Graafl accelerators, the gas which is fed to the ionsource at the high-voltage end of the accelerating tube is pumped outvia the latter tube, which therefore serves the double function ofaccelerating tube and vacuum duct. The form of tube suitable foraccelerating particles, however, is not the most suitable for a purelypumping function, because the electrode structure used necessarilyincreases its impedance to gas flow. An alternative arrangement is toprovide a second tube, effectively arranged in parallel with theaccelerating tube, whose sole function is to extract gas from the ionsource region. This second tube is known as a differential pumping tube.A tandem accelerator using two such diflerential pumping tubes isdescribed, for example, in the Review of Scientific Instruments, volume35, Jan. 1964, page 1 et seq.

A well known phenomenon in electrostatic accelerators is electronloading, in which electrons are accelerated back towards the positiveend of the tube. The accelerated electrons, which can originate invarious ways, e.g. by field emission from the electrodes, generateX-rays when they strike parts of the accelerator structure, which inturn ionise the high-pressure gas surrounding the structure and soreduce the insulation. Moreover the electrons themselves constitute anadditional load on the generator, tending to reduce its output voltage.

Various forms of accelerating tube have been proposed to suppress theelectron loading effect. These are usually designed to deflect theelectrons into the tube structure before they have acquired sufiicientenergy to have a serious effect. It is an object of the presentinvention to provide a differential pumping tube which combines a highdegree of electron suppression with low impedance to gas fiow.

According to the present invention a difl erential pumping tubecomprises a plurality of annular electrodes spaced apart by annularinsulators, each electrode including a substantially diametral barwhereof the two radial halves are deflected from a plane normal to thetube axis towards one end of the tube to produce an electric fieldbetween adjacent electrodes having a component directed towards the tubeaxis.

Preferably the inner periphery of each electrode is also deflected fromsaid plane to enhance said field as aforesaid.

Preferably successive electrodes are arranged so that each bar iscircumferentially displaced by a small angle relative to the bar of thepreceding electrode to provide optical baflling along the tube.

The bars may be deflected to V-shape or arcuate form.

To enable the nature of the present invention to be more readilyunderstood, attention is directed, by way of example, to theaccompanying drawing, wherein FIGURE 1 is a cross-sectional elevation ofpart of a pumping tube embodying the present invention.

FIGURE 2 is an axial view looking through the tube of FIGURE 1.

FIGURE 3 shows a modification of the tube of FIG- URE 1.

Referring to FIGURE 1 a pumping tube comprises a plurality of aluminumelectrodes 1 (of which only two are shown complete) separated by annularglass insulators 2 and having an inner periphery 3. Extendingdiametrically between two points on the inner periphery of eachelectrode is a bar, the two radial halves 4a and 4b of which aredeflected from the plane of the electrode (which is normal to the tubeaxis) to meet in a V on the tube axis. The effect is to produce anelectric field between adjacent electrodes, indicated by the arrows,which has a component directed towards the tube axis. This componentcauses any electrons within the tube to impinge on the bars instead oftravelling along the tube parallel to the axis.

In peripheral regions remote from the bar, the radial component ofelectric field might be insufficient to prevent some movement ofelectrons parallel to the axis. To enhance the radial component in theseregions, the entire inner periphery is itself deflected, atapproximately the same angle as the two halves of the bar.

The bars are made sufficiently narrow to ofier little impedance to theflow of gas along the tube. To produce even more effective electronsuppression, successive electrodes are mounted with their barscircumferentially displaced through a small angle relative to thepreceding bar to produce an optical bafiling effect looking along thetube axis. The eflect is illustrated in FIGURE 2, where the angulardisplacement between successive bars is 15'. It will be seen that withthe dimensions shown, no electron travelling parallel to the tube axiscan traverse more than eleven electrodes without impinging on a bar.

FIGURES 1 and 2 illustrate the described embodiment approximately toscale, the diameter of the edge of the inner periphery being 5% inches.The bars are inch wide at the periphery, tapering to /2 inch at theaxis. The electrodes are spaced 1.1 inches apart by the insulators.

In another embodiment, as shown in FIGURE 3, the diametral bars 4 havethe same dimensions as in FIG- URES 1 and 2 and are circumferentiallydisplaced by the same amount. However instead of being deflected to aV-shape, the bars 4' are arcuate in form, the centre of the are (whereit intersects the tube axis) being displaced about 1% inches from theplane of the electrode. An example of this embodiment, comprising 136such electrodes spaced 1.1 inches apart, has been used at voltages up to5.5 mv. with no measurable electron loading. The pumping impedance ofthis tube with air was approximately 28 litres/sec.

We claim:

1. In a differential pumping'tube for evacuating the high-voltageportion of an electrostatic accelerator in parallel with theaccelerating tube and comprising a plurality of annular electrodesspaced apart by annular insulators, the improvement wherein eachelectrode includes a substantially diametrical bar which intersects theaxis of the tube, the two radial halves of said bar being deflected froma plane normal to the tube axis towards one end of the tube to producean electric field between adjacent electrodes having a componentdirected towards the tube axis to cause deflection of unwanted electronsinto said electrodes.

2. A tube as claimed in claim 1 wherein the inner periphery of eachelectrode is deflected from said plane towards said one end of the tubeto enhance said electric field.

References Cited UNITED STATES PATENTS 2,570,158 10/1951 Schissel 3l3362,917,630 12/1959 Walbank 313-61 3,174,678 3/1965 Matricon 230-693,175,373 3/1965 Holkeboer et a]. 230--69 JAMES W. LAWRENCE, PrimaryExam ner.

20 C. R. CAMPBELL, Assistant Examiner.

1. IN A DIFFERENTIAL PUMPING TUBE FOR EVACUATING THE HIGH-VOLTAGEPORTION OF AN ELECTROSTATIC ACCELERATOR IN PARALLEL WITH THEACCELERATING TUBE AND COMPRISING A PLURALITY OF ANNULAR ELECTRODESSPACED APART BY ANNULAR INSULATORS, THE IMPROVEMENT WHEREIN EACHELECTRODE INCLUDES A SUBSTANTIALLY DIAMETRICAL BAR WHICH INTERSECTS THEAXIS OF THE TUBE, THE TWO RADIAL HALVES OF SAID BAR BEING DEFLECTED FROMA PLANE NORMAL TO THE TUBE AXIS TOWARDS ONE END OF THE TUBE TO PRODUCEAN ELECTRIC FIELD BETWEEN ADJACENT ELECTRODES HAVING A COMPONENTDIRECTED TOWARDS THE TUBE AXIS TO CAUSE DEFLECTION OF UNWANTED ELECTRONSINTO SAID ELECTRODES.